First responders and volunteers who helped with 9/11 recovery efforts have only a month left to register for future workers’ compensation benefits in case they fall ill.

More than 20,000 people could be eligible, but they must register before Sept. 12.

The registration effort is part of the New York State Workers’ Compensation Board’s “Tell Us You Were There” campaign, designed to protect those who helped after the World Trade Center attacks.

“Most people are eligible. You should file a WTC-12 form whether you were injured or not and whether you were employed or volunteered,” the board said. “This preserves your right to future benefits, should you ever need them.”

Previous legislation guaranteeing the workers’ comp expired Sept. 13, 2010. But last year, Gov. Cuomo signed legislation that extended the deadline and expanded the list of covered illnesses.

Now WTC workers or volunteers can get benefits if they develop psychological ailments or illnesses of the upper or lower respiratory or gastroesophageal tracts.

The WTC-12 registration form is available at www.wcb.ny.gov/WTC12. Anyone with questions can call (855) WTC-2014.

In addition to Ground Zero, qualified applicants can have worked at the Fresh Kills Landfill, on barges or piers or at morgue sites — as long as it was before Sept. 12, 2002.

So far, 40,737 people have filled out the form, said Joe Cavalcante, a compensation-board spokesman.

There have been 5,165 workers’ comp claims paid to those who took part in 9/11 rescue or recovery efforts, he said.